Frequency modulation television receiver



Sept- 5, 1949- lv. J. DUKE ETAL 2,480,913

FREQUENCY MODULATION TELEVISION RECEIVER Filed Dec. 27V, 1943 4Sheets-Sheet` 2 ucl/Vm osc/zMra/a 90m PM INVENToRs. 'e s si vee/vow' J.puffs,

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FREQUENCY MODULATION TELEVISION RECEIVER Filed Dec. 27, 1943 4Sheets-Sheet 3 was@ LF. uw o 60 Q l l,.5

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.late the audio frequency carrier.

Patented Sept. 6, -1949 UNITED sTATEs PATENT orti-,cs

FREQUENCY MODULATION TELEVISION RECEIVER Vernon J. Duke, RockvilleCentre, Y., and Robert W. Clark, Teaneck, N. J., assignors to RadioCorporation of Americana corporation vof 'Delaware Application December27, 1943, Seria1'No..515,818

4 Claims. 1

This invention relates to an improvement 'in television systems, andmore .particularly to an improvement in a television receiver forreceiving a radio frequency carrier which has `been frequency modulated.by the television signals.

In conventional xtelevision systems the video or .image `and.synchronizing signals are transmitted as modulations of one carrierfrequency while the accompanying audio or ,sound signals are transmittedas a modulationrof a .separate carrier, gen- .erallypositioned-slightlyVabove the video or image carrier. It is normal practice .to amplitude.mod-

ulate .the video carrier and to frequency .modu- In the amplitudemodulation .of the radio `frequency .carrier by the video signal, it isstandard practice to control the .carrier amplitude insuch a way .that.full carrier amplitude -(or 100%.) .is .transmitted during the presenceof synchronizing signals only. Full blackrin the subject is generallytransmitted at approximately 7.5% of maximum `carrier arnplitude, andfull white in the ,picture is .normally transmitted at .approximatelyzero .amplitude of the carrier (when direct :current insertion isemployed). vnecessary in television in :order that the 'background oraverage light :intensity maybe transmitted to the receiver .and in.order that vthe .Synchronizing signals may .be .separated from vtheimage signals by .amplitude .selection or .other .appropriate means.

The .audio signals are .generally transmitted as ,a .frequencymodulation .of the .carrier lprovided for that purpose,.and are.detected 'by `appropriate discrimination vcircuits lin the televisionreceiver. Since .the .audio .and video signals are .transmitted asmodulations ,ofseparate carriers, itis com- :mon :practice ,in .atelevision .receiver to .employ .a .single local oscillator, which whenbeat .against the two received radio frequency carriers `produces -twoseparate intermediate frequencies. One of these is a frequencyYmodulated intermefdiate .frequency carrier and includes Ythe .audioinformation, whereas .the other ,is an `amplitude modulated intermediateifrequency .carrier and yincludes the video information.

In .the present .invention provision .is made whereby .both the videoandthe .audio signals :are transmitted -by .frequency modulation .oflseparate v`radio frequency carriers,.and when-.this is done inaccordance with the present invention, the Yconventional video vsignalpresent :at dthe .transmitter -is -used A`for frequency `.modulating an.appropriete carrier.

In the present invention `two .different .typesof Signal transmission in-this .manner iisreceiving circuits may .be employed. In one nstance the,-full frequency .deviation produced by .the modulation video signal isrepresented by Aa sing-le intermediate frequency channel and a ,single,discriminator is usedso that `at the .output of `the :discriminator aIcomplete composite television signal will be present including .theimage signals, the blanking andthe synchronizing signals.

-Inanotlier form of the present ,inventionel .ara-te intermediatefrequency Achannels are employed in the receiver, one lfor passing the`frequency `-deviations representing the image portion .of the receivedsignals and the .other .for passing .only the frequency deviationsrepresenting the synchronizing portion of the received signals. :In thisparticular form of .the present invention, the complete composite videosignal is `received asa frequency modulation of a single carrier, .butby providing sepa-rate intermediate .frequency channels having differentband pass characterristics, the image signals and the synchronizingsignals may be separated `.according to the frequency deviations of theVintermediate frequency gcarrier. When such la system is employed, twoseparate detectors or discriminators are used, -oneor responding.to-,the `frequencydeviations-ocfcurring within lthe `image intermediatefrequency channel and the other for responding to frequency deviations.occurring .within the ,synchronizing intermediate ,channel frequency.When separate intermediate frequency channelsare provided Vfor itheimage ,and synchronizing signals, `complete and ,convenientseparation ofthesesignals is lpossible. It is, therefore, .possible to cause ,thereceiver .to respond .to .synchronizing impulses of lproper amplitude-while at the same time there- .ceiver is unresponsive to transient`conditions or .undesired disturbances which produce deviations lessrthan .maximum ,deviations produced by thesynchronizing impulses.

Inthe present .invention the audio signals .may .still vbe transmitted.asa frequency ,modulation of .a separate radio frequency carrier, andfor the .transmission of these signals through vthe .reoeiver .aseparate .intermediate frequency ampliier `channel may lbe provided.

.It is, therefore. .one purpose of the present in- .vention .to lprovidemeans `for Vtransmitting video .signals .as airequency modulation of aradio frequency carrier.

Another :purpose fof the present invention ,re- .sides in .the.provisionof .a television receiver responsiveto .frequencymodulationsof .a radio frequency ,carrier in which the ,frequencymodulations are produced in accordance with the television videosignals.

Still another purpose of the present invention resides in the provisionof a television receiver for receiving a frequency modulated carrier,the frequency deviations of which are produced in accordance withcomposite video signals, together with means for detecting andseparating the image signals from the synchronizing signals.

A still further purpose of the present invention resides in theprovisions of means in a television receiver for receiving a frequencymodulated radio frequency carrier in which the frequency deviation-s areproduced by a composite video signal and in which the receiver includestwo intermediate frequency channels, the band pass of one beingsuincient to accommodate the frequency deviations representing the imagesignals, and the band pass of the other being chosen to accommodate thefrequency deviations representing the synchronizing signals.

Still another purpose of the present invention resides in the provisionof means in a television receiver for receiving a video signal frequencymodulated radio frequency carrier in which the various intermediatefrequency band pass circuits are so designed that over-modulation orexcessive frequency deviation do not produce any undesired or erroneousresults.

Various other advantages and purposes of the present invention willbecome more apparent to those skilled in the art from the followingdetailed description, particularly when considered in connection withthe drawings, wherein like reference numerals represent like parts andwherein:

Figure 1 diagrammatically represents one form of the present invention;

Figure 2 shows a graph and curves used in explaining the operation ofthe system shown in Figure 1;

Figure 3 shows a diagrammatic modification of the present invention;

Figure 4 shows a graph and curves u-sedin explaining the operation ofthe system shown in Figure 3;

Figure 5 shows curves representing one undesired result that will takeplace under one set of circumstances;

Figure 6 shows curves representing another undesired result that willtake place under another set of circumstances; and

Figure 7 shows curves representing desired response under conditions ofover-modulation.

Referring now to Figure 1, there is shown schematically a televisionreceiver including a radio frequency tuner l0 to which signals receivedupon an appropriate antenna I2 (or by way of other communicationchannel) are supplied. A local oscillator I4 is also provided, thefrequency of operation of which may be altered in accordance with theradio frequency tuning of the receiver. The received signals and theoscillations from the oscillator I4 are supplied to a mixer in order toproduce an intermediate frequency carrier which is applied to anintermediate frequency amplifier i6. If the audio signals whichaccompany the video signals are transmitted as modulations of a separateradio frequency carrier, the audio intermediate frequency carrier may besupplied to appropriate amplifiers and demodulators over conductor I8. YY

Since the present invention is not concerned with the particular audiosystem employed, details of that system are not shown herein.

` video signal series.

The output from the video intermediate frequency amplifier is thenapplied to a discriminator network 20 (preceded by a limiter, ifdesired), and the resulting video signals are, in turn, applied to anappropriate video amplifier 22. The discriminator also applies signalsto a synchronizing signal separator 24 in order that the synchronizingsignals may be separated from the image signals thereby to control theoperation of horizontal and vertical deflection generators 26. Theamplified video signals as supplied by the amplifier 22 are applied tothe control electrode of an image producing tube or Kinescope 28 inorder to modulate the current intensity of a cathode ray beam developedtherein. In order that the cathode ray beam may be caused to scan atarget surface or luminescent or fluorescent screen, energy from thehorizontal and vertical deflection generators 26 is applied to adeflection yoke 30 associated with the image tube.

As stated above, the television receiver is designed to receive a radiofrequency carrier that has been frequency modulated by the composite Inorder to clearly explain the operation of the system represented byFigure 1, reference is now made to Figure 2 of the drawings.

It will be assumed, for example, that the center frequency of the radiofrequency carrier is 104 megacycles, with deviation limits extending 4:megacycles on each side of the carrier center as shown in Figure 2.Therefore, the lower deviation limit of the carrier is approximately 100megacycles, whereas the upper deviation limit is 108 megacycles.

It will also be assumed that the peaksV of the synchronizing impulsesare transmitted at approximately the upper deviation limit and that fullwhite in the image is transmitted at approximately the lower deviationlimit. If a standard composite video signal series is used to frequencymodulate the radio frequency carrier, then full black in the image willbe represented by 106 megacycles. Under these conditions a band of 6megacycles is provided for the transmission of the image signals and,similarly, a band of 2 megacycles is provided for the transmission ofthe synchronizing signals.

If under these conditions the local oscillator at the receiver operates,for example, at 90 megacycles, a complete intermediate frequency band of8 megacycles is required for the transmission of the intermediatefrequency carrier, and this band extends from 10 megacycles as a lowerdeviation limit to 18 megacycles as an upper deviation limit. Full blackin the image (or the blanking impulses) occurs at 16 megacycles in theintermediate frequency channel in the assumed exemplary system.

For responding to these frequency deviations a discriminator must beprovided having a suflicient frequency band width to respond to the full8 megacycle deviation band in a substantially linear fashion. A desireddiscriminator response curve is shown in Figure 2, and when frequencydeviations from the intermediate frequency amplifier channel are appliedto the discriminator, a series of composite video signals such asrepresented in Figure 2 may be produced. It is desirable that thediscriminator respond substantially linearly to frequency deviationsoccurring between 10 and 18 megacycles, and it may also be desirableunder some circumstances that the discriminator be non-responsive tosignals occurother, provided no over-modulation occurs'.

ring below megacycles or above' 18 megacycles.

A discriminator having such a response curve is shown and described inU. S. Letters Patent No. 2,413,913, issued'to Vernon J. Duke on Jan.-'7,

It may be seen, therefore, that when a television receiver such as thatrepresented in Figure 1 is providedfor the'reception of a frequency'modulated carrier, a composite video signal may loe-produced, as shownin Figure 2, which includes the image signals, the blanking signals andthe sy'n ':hronizing'V signals in theirproper intensity, and havingproper values with respect to each To produce such a result it is'merelynecessary to frequency modulate a radio frequency carrier by thepresently use d video signals, and when such signals are received andare applied to an appropriate discriminator, a video signal series willbe produced which may be employed for producing a-'visual image in amanner similar to the present amplitude modulation receivers.

In a system as describedabove, some form of 'synchronizing separationcircuit must be provided for separating the synchronizing impulses fromthe image signals and, in addition, a relatively wide bandintermediatefrequency amplifier channel and discrimnator must be provided.

In a modification of the present invention it is proposed' to providetwo separate intermediate frequency channels in the receiver (inaddition to the sound channel), the channels being related to each otherin such aY manner that a single radio vfrequency carrier that has beenfrequency modulated by the video signal may be received. In thisrespect, reference is now made to Figure 3 wherein a receiver diagram isshown which includes a radio frequency tuner and mixer Ill as in Figure1 to which is applied energy from an appropriate antenna I2, forexample. An oscillator I4 is also provided for heterodyning with thereceived carrier signals to produce intermediate frequency carriers. Thefrequency deviations which are produced as a result of the tele-A'vision image signals are `then applied to the image vturn suppliesimage signals to an appropriate limagef'signal amplifier 46. Thesynchronizing in'- termediate frequency Vamplifier supplies signals toa'syn'chronizing signal discriminator 48 (also after' limiting, ifdesired) and the output of this discriminator isapplied as controlsignal impulses uponv the horizontal and vertical deection generators,conventionally shown at 5U. The v horizontaland vertical deflectiongenerators sup- Aply energy to a deflection yoke 36 for properly de-"iiecting a cathode ray beam developed within the Kinescope or imageproducing tube 28, and the rier is assumed, for example, to be 104meacycles. v l

Since the radio frequency carrier has a maximum deviation of 8megacycles, the lower deviation limit is approximately 100 megacycles,whereas the upper deviation limit is approximately 108 megacycles. Sincefull vblack in the image (or the blanking impulses) normally has anintensity corresponding to'75%V of the intensity of the'peaks of thesynchronizing impulses, 'the frequency corresponding to full blackvwillb'e 106 megacycles.

With such a received radio frequency carrier (which 'corresponds to thereceived carrier inthe conditions assumed in Figure 2), if a radiofrequency oscillator operating at megacycles -is provided in thereceiver, a frequency deviation of from 10 to 18 megacycles will beproduced at the output of the mixer l0. At this point in thecircuit twoseparate band pass amplifiers are provided, one of which is designed toaccommodate the frequency deviations representing the image signalswhich extend from 10 to 16 megacycles, the other band pass amplifierbeing designed to accommodate at least a portion of the frequencydeviations representing the synchronizing signals. These deviationsextend froml 16 to `18 Vmegacycles. The rst intermediate frequencyamplifier 46, therefore, passes aV band of frequencies extending from 10to 16 megacycles, and after these signals have been properly amplified,they are then applied to an image discriminator which has vacharacteristic such as that represented in Figure 4 andV which respondsto the image signal deviations. At the output of the image discriminatorimage signals may then be derived which may have a wave form such Vasthat represented by curve 52 in Figure 4.

It will be observed that the output from theimage Ydiscriminator 44 doesnot include any syn- 'chronizing impulses, but includes onlyV thosesignals which are representative of the particular television imagefrequency deviations. The frequency deviationsrthat extend above, 16megacycles and whichare a result of the transmission Vof thesynchronizing signals are notimpressed on Acycle deviation band from '16to y18 megacycles,

but may have a narrower band of. approximately one megacycle preferablychosen to extend from A1f7 to 18 megacycles as shown in Figure 4. Underthese conditions f the synchronizing signal discriminator 48 need onlybe responsive to signals occurring within these limits, and such adiscriminator may have a characteristic as that representedin Figure 4.Atthe output of the synlchronizing signal discriminator are thenavailable 'the synchronizing signals onlyasA represented by curve 54 inFigure 4; l

There is a distinct advantage in limiting the band' Width ofAthesynchronizing signal intermediate frequency amplifier 42 tofrequencies" occurring within the band of from 17 to 18 megacycles,since noise disturbances caused by static,

Yautomobile ignition, etc., are therefore at least in part renderedineffectual since the synchronizing signal band pass amplier 42 isunresponsive to .deviations occurring below 1 7 megacycles or above .18megacycles. Since all synchronizing impulses :By using a system such asthat described above .in connection with Figures 3 and 4, it maytherefore beseen that a radio frequency carrier that has been frequencymodulated by a standard vcomposite video signal series may beconvenient- .lyreceived and applied to two separate intermediatefequency amplifier channels to result in lcomplete and convenientseparation of the synchronizing signals from the image signals.

Furthermore, under these conditions the band `Width, of the intermediatefrequency amplifier Vchannels need not be excessively large, and,furthermore, the discriminators are not required to respond to anexcessively wide frequency deviation band. The output from the imagedis- `crirninator as represented by curve 52 in Figure 4 Ymay. beapplied directly to an appropriate image amplifier 46, and aftersufficient amplification the signal variation may then be applied to thecontrol electrode of the image producing tube. No synchronizingseparation circuits are necessary, Ysince the separation is produced asa result of proper choice of intermediate frequency bands with theresult that the output from the synchronizing signal discriminator 48may be applied directly to the horizontal and vertical deflectiongenerators used in the receiver.

The aboveassumed frequencies are for the purpose of explanation only,and it is obvious that .any other range of frequencies or frequencydeviationy limits may be employed.

In explaining the systems shown in Figures 1 Aand 3, it is assumed thatno frequency deviation foccurs outside the prescribed deviation limits.

Under actual operation it has been found that frequency deviationsbeyond the desired limits frequently occur, and in a system having bandpass limits such as described above in connection with Figures 2 and 4,certain undesired effects will result. In order to describe theseundesired effects, reference is now made to Figure 5 which shows afrequency discriminator response curve having substantially linearresponse characteristics between frequencies extending from fn to f1.'The response of the discriminator falls off rapidly for frequenciesbelow fn or above f1. With such a discriminator it is assumed that thevideo intermediate frequency band pass amplifier has a responsecharacteristic such as that represented at Ell, which, as may be seenfrom Figure 5, extends beyond the response band of the discrirninator.If frequency deviations such as represented by curve 62 are applied tothe intermediate frequency amplifier and discriminator, a Voltagevariation such as represented at 64 is available at the output of thediscriminator. It will be noticed that the height of thesynchronizingsignal for curve 62 does not extend beyond frequency, f1,nor do any signals representingwhite extend below fo. In other words,the applied frequency deviation as represented by curve 62 is withinthe'response limits of the discriminator, with the result that lnodistortion occurs and a proper voltage varia- Cil tion represented bythe Vcurve 64 will be produced at the output of the discriminator.

. If, however, deviations beyond the limits of the discriminatorarereceived at the receiver as represented by curve 66 of Figure 5, thencertain undesired effects will result. If, for example, thesynchronizing signal should for some reason deviate beyond frequency f1as represented by point A of curve 66, the discriminator output will notcorrespond to the applied frequency deviation. This occurs by reason ofthe fact that the deviations extending beyond frequency fi, althoughwholly within the band pass limits of the inter- /mediate frequencyamplifier, are beyond the response characteristics of the discriminator,with the result that the output of the discriminator drops to zerorather than being a value such as .represented by A of Figure 5. Thedotted portion .output from a discriminator having a characteristicrepresented by ,curve 58 of Figure 5. Since .the output of thediscriminator falls to zero, such a signal potential would not besuitable for syn- ,chronizing purposes, but, instead, would correspondto a picture signal representative of gray.

Similarly, if the deviation inthe direction of white extends below thedeviation limit fo, then the dis- Vcriniinator response is such that theoutput signal is driven in the direction of black, as represented by thesolid line in curve 68, rather than remaining .at point B', or at avalue corresponding to full white in the image.

It will be seen, therefore, that if a television receiver employs anintermediate frequency band pass having suicient width to accommodateappreciably more than the normal frequency deviation, llout if thediscriminator response will .accommodate only the theoretical orprescribed .frequencydeviatiom serious distortion of the wave formderived from the discriminator will result if the frequency deviation ofthe signal remains within the intermediate frequency amplifier bandwidth but extends beyond the discriminator frequency limits.

A similar undesired distortion results, as shown in Figure 6, when thediscriminator response is of sufficient width to accommodate excessivedeviations, but where the video intermediate frequency band amplifierhas a frequency limit only suiiicient` to accommodate the normal orprescribed frequency deviation. In this particular instance it will beassumedrthatthe video intermediate frequency band accommodates onlyfrequencies extending from fo to fr, as represented by curve 'I inFigure 6. Itwill also be assumed that the discriminator has a widerdeviation response ranging from below frequencies corresponding to foand to above frequencies corresponding to f1. The curve of such aresponse is indicated at 12 in Figure 6.

If, under these conditions, the applied frequency deviations remainwithin the band from fo to f1, as represented by curve 74, then acorresponding and proper output voltage variation, Vas represented bycurve 16, may be derived from the discriminator, since both theintermediate frequency amplifier and the discriminator will accommodatesuch a frequency deviation. Should the frequency deviation extend beyondthese limits, however, as represented by curve 18 of Figure 6,distortion will occur, as shown in curve 80, since the voltage variationfrom the discriminator will be that represented by the solid rather thanthe dotted portion of curve 88. Should the synchronizing impulse extendbeyond f1, as for example point C in Figure 6, then the output from thediscriminator, instead of extending beyond black in the image as shouldbe the case, will actually extend from biack in the direction of whiteto a value corresponding to approximately zero Voltage from thediscriminator. The dotted portion C" shown at Figure 6, representing thesynchronizing signal, will therefore be eliminated, and in the absenceof such synchronizing signal the receiver may drop out ofsynchronization with the transmitter.

A similar situation will result if the frequency deviation in thedirection of white in the picture extends below frequency fu, and assoon as the deviation extends beyond fo the voltage variation in theoutput from the discriminator will be abruptly driven toward zero ratherthan remaining of a value corresponding to white, or extending abovewhite as shown at D', inasmuch as the band pass limits of theintermediate frequency amplier channel have been exceeded.

Accordingly, if the video intermediate frequency band pass amplifier hasa band width only sufficient to accommodate the normal frequencydeviation limits, serious distortion will result should the deviation,for some reason, extend beyond these limits.

Since excessive frequency deviations cannot be entirely prevented, someprovision must be made in the receiver for preventing serious distortionwhen such excessive deviation occurs. Obviously,

if either the discriminator (as shown in Figure 5) or the intermediatefrequency amplifier (as shown in Figure 6) will accommodate onlyfrequencies occurring within the normal deviation band, distortion suchas represented in Figures 5 and 6 will result. Such distortion may causeloss of synchronism, and where frequencies representative of white'erroneously produce signals representative of black or gray in theimage, considerable visible distortion will result in the producedtelevision image.

In order to guard against such distortion, a receiver with componentshaving response characteristics such as represented in Figure '7 may beused. In this figure it will be assumed that the normal deviation bandextends from fo to f1 for the complete video signal series. In thissystem two separate intermediate frequency amplifier channels are used,as shown and described in connection with Figures 3 and 4, with thenormal image deviation extending from fu to f2 and the synchronizingsignal normal deviation extending frcm ,f2 to f1. Since frequency f2 isrepresentative of black, the image intermediate frequency amplifier neednot respond to any frequency above f2. Accordingly, the upper frequencylimit of the image intermediate frequency ampliier band may coincidewith frequency f2 as indicated by curve 84 in Figure 7. Frequencydeviations below frequency fo which correspond to full white in theimage should be passed by both the intermediate frequency amplifier andthe image discriminator in order to avoid the distortion represented atB' and D in Figures 5 and 6, respectively. Accordingly, the lowerfrequency limit of the image intermediate frequency amplier shouldextend an appreciable distance below fo.

The image discriminator may have a response such as indicated at 86' inFigure 7, 'and it is l@ preferable that the response be substantiallylinear between the limits of fo and f2. The response of thediscriminator for frequency deviations above f2 should remain at a valuecorresponding to the response at frequency f2 to assure th'e productionof a signal corresponding to full black in the image even thoughfrequencies above f2 cannot be passed through the image intermediatefrequency amplifier, The discriminator should be responsive to signalsbelow fo, and in order to prevent blooming on the image screen of thereceiver, it is preferable that the response be maintained uniform for apredetermined amount below frequency fu, as indicated by curve 86.Accordingly, when frequency deviations representative of image` signalsare applied to an intermediate frequency amplifier havingcharacteristics indicated by curve 34 and to a discriminator havingcharacteristics indicated by curve 86, the distortion shown in Figures 5and 6 will not occur. A. frequency deviation such as represented bycurve 88 of Figure '7 may bie applied to these elements, and even thoughthe deviation may sporadically extend below fo as inicated at E, thevoltage output from the discriminator will remain at full White asrepresentedv at E in curve 93, rather than drop to some value in thedinection of black as in the case of Figures 5 and 6. Similarly, thepresence of frequency deviations beyond fz (as occur during thesynchronizing intervals) does not cause any change in the output signalstrength from a value corresponding tofull black.

In the synchronizing signal intermediate frequency amplifier it is notnecessary that any frequencies below f2 be passed since the frequency f2is representative of full black and all synchronizing signals extend ina direction above frequency f2. Accordingly, the synchronizing signalintermediate frequency amplifier may have a nesponse such as indicatedby curve 92 in Figure 7, with frequency f2 representing the lowerfrequency limit and with the upper frequency limit extending appreciablyabove f1, in order to accommodate any excessive frequency swing in thatdirection.

Similarly, the synchronizing discriminator response may be such asrepresented by curve 94 in Figure 7, and should preferably besubstantially linear between the limits of f2 and f1, and should also beresponsive to frequency deviations above f1 in order to preventelimination of the synchronizing signals as is the case in conditionsrepresented by Figure 5 or Figure 6. The output fromthe discriminatorshould remain unchanged (at zero, for example) for deviations below fz,and since frequencies below f2 cannot pass through the synchronizingsignal intermediate frequency amplier, as indicated at 92, the outputpotential level of the discriminator, in the absence of an appliedsignal, should correspond to its output when frequency f2 is appliedthereto.

If the frequency variations remain within the ,l limits of f2 and f1,then a normal synchronizing signal will be developed as represented bycurve S6, and also if the deviation should exceed frequency f1 asindicated at F in Figure 7, a proper synchronizing signal will still bederived from the discriminator since such excessive deviation is stillwithin the pass band of the intermediate frequency amplifier and sincethe discriminator will respond, with substantially uniform output, tosuch excessive deviation. Accordingly, an excessive deviation in thedirection of the synchronizing signal Will still produce a voltagevariation at the output of the discriminator corresponding to the signalproduced by a normal deviation.

Intermediate frequency amplifiers having the desired band width andcharacteristics may readily be designed in a manner well known to thoseskilled in the art. Furthermore, frequency discriminators of the desiredband width and having characteristics such as represented by curves 86and 94 in Figure 7 may also be provided by appropriate design and may,in fact, form a part of the intermediate frequency amplifier channels.For example, the image intermediate frequency amplier channel may bedesigned as a low-pass iilter having uniform response to frequenciesbelow fo, and having a linear attenuating effect for frequencies betweenfe and fz. The response should be zero for frequencies above f2. Whensuch an amplifier channel is provided, a frequency discrimination effectresults between ,fu and f2. For the synchronizing intermediate frequencyamplifier channel a high-pass filter circuit may be employed having zerotransmission eiiiicency (or cut-oli) below f2, a linear increase inresponse from f2 to f1, and a luniform output response for frequenciesabove f1. Amplitude modulation limiters may or may not be used in such asystem, as desired.

When a television receiver is provided with image and synchronizingsignal intermediate frequency band pass amplifiers such as representedby curves 84 and 92 in Figure 7, and when appropriate discriminators orfrequency discriminating amplifiers are provided having characteristicssuch as represented by curves 86 and 94, it is possible to derive fromthe discriminator or frequency discriminating amplifier channels animage signal such as represented by curve 90 and a synchronizing signalsuch as represented by curve 96 without the distortion associated withcircuit elements such as represented in Figures 5 and 6. The clipping"action produced at E and F under conditions of excessive frequencydeviation is desirable, since in the image signal such clipping actionprevents blooming or the production of excessive white on the receiverscreen as stated above.

Furthermore, the clipping action exercised on excessive deviation in thedirection of the synchronizing signals is desirable, since thevoltagevariations derived from the synchronizing signal discriminator remainssubstantially uniform in intensity regardless of such excessivefrequency deviation.

With the present invention it is, therefore, apparent that a televisionreceiver may be provided which will respond to a frequency modulation ofa radio frequency carrier where the frequency modulation is produced bya conventional composite video signal. It is also possible to utilizesuch a frequency modulated signal Without resulting in undesireddistortion and possible elimination of the transmitted synchronizingsignals.

By exercising the present invention in conjunction with frequencymodulated transmission of the accompanying audio signals, the televisiontransmission may be made completely frequency modulated with desirableand satisfactory results at the receiver.

Although the present invention is described somewhat in detail, variousalterations and modiiications may be made therein without departing fromthe spirit and scope thereof, and it is desired that any and all suchmodifications be considered within the purview of the present inventionexcept as limited by the hereinafter ap-N pended claims.

Having now described our invention, what we claim as new and desire tohave protected by Letters Patent is:

1. A television receiver for receiving a frequency modulated radiofrequency carrier modulated by television video signals such that thefrequency deviations representative of the image signals and thefrequency deviations representative of the synchronizing signals occupyadjacent bands, comprising an image signal band pass amplifying channelhaving characteristics such that frequency deviations representative ofthe image signals may be amplifier thereby, a frequency discriminatorassociated with said image signal amplifying channel, said discriminatorhaving substantally linear frequency discriminating characteristicsthroughout substantiallythe entire image signal frequency deviation bandand having substantially uniform response to frequencies outside theimage signal frequency deviation band whereby television image signalsmay be produced by said discriminator, means for r applying the producedimage signals to a translating device, a synchronizing signal band passamplifying channel having characteristics such that frequency deviationsrepresentative of the synchronizing signals may be amplified thereby, asecond frequency discriminator associated with said synchronizing signalamplifying channel, said discriminator having substantially linearfrequency discriminating characteristics over at least a portion of thesynchronizing signal frequency deviation band and having substantiallyuniform response to frequencies outside the synchronizing signalfrequency deviation band whereby television synchronizing signals may beproduced by said second discriminator, and a synchronizing signalresponsive circuit associated with said second discriminator.

2. A television receiver for receiving a frequency modulated radiofrequency carrier modulated by television video signals such thatthefrequency deviations representative of the image signals and thefrequency deviations representative of the synchronizing signals occupyadjacent bands, comprising an image signal amplifying channel havinglow-pass characteristics with uniform response below a predeterminedcarrier frequency corresponding to one frequency deviation limit of theimage signals and with substantially linear diminution of response inthe image signal frequency deviation band whereby at the output of saidamplier a series of image signals may be produced, a synchronizingsignal amplifying channel having high-pass characteristics with uniformresponse above a predetermined carrier frequency corresponding to onefrequency deviation limit of the synchronizing signals and withsubstantially linear diminution response in the synchronizing signalfrequency deviation band whereby at the output of said syn- 1 chronizingsignal amplifier a series of synchronizing signals may be produced, andsignal utilizing elements coupled to each amplifier circuit.

3. A television receiver for receiving a frequency modulated radiofrequency carrier modulated by television video signals such that thefrequency deviation band representative of image signals is adjacent thefrequency deviation band representative of synchronizing signals,comprising a band pass image signal amplifying channel having a slopingcharacteristic such that substantially linear frequency responsediscrimination is present within the image signal frequency deviationband whereby ai; the output of said amplier a series of image signals isproduced, a band pass synchronizing signal amplifying channel having asloping characteristic such that substantially linear frequency responsediscrimination is present within the synchronizing signal frequencydeviation band whereby at the output of said synchronizing signalamplifier a series of synchronizing signals is produced, and translatingdevices individually responsive to the produced image and synchronizingsignals.

4. A television receiver for receiving a frequency modulated radiofrequency carrier modulated by television video signals such that thefrequency deviation band representative of image signals issubstantially adjacent the frequency deviation band representative ofthe synchronizing signals, comprising a first amplifying channel havinglow-pass characteristics with uniform response below a predeterminedcarrier frequency corresponding to the low frequency deviation limit ofone frequency deviation band and with substantially linear decrease infrequency response within said one frequency deviation band whereby atthe output of said ampliiier a first series of signals is produced, asecond amplifying channel having high pass characteristics with uniformresponse above a predetermined carrier frequency corresponding to thehigh frequency deviation limit of the other frequency deviation band andwith substantially linear increase in frequency response within saidother frequency deviation band whereby at the output of said secondamplifier a second series of signals is produced, and signal utilizingelements coupled to each amplifier circuit.

VERNON J. DUKE.

ROBERT W. CLARK.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Number Name Date 2,254,435 Loughren Sept. 2, 19412,290,517 Wilson July 21, 1942 2,413,913 Duke Jan. 7, 1947 2,435,736Carnahan Feb. 10, 1948 OTHER REFERENCES Carnahan F-M Applied to aTelevision System, Electronics, Feb., 1940, pages 26, 30, 31 and 32.

Duplex Transmission of Frequency-Modulated Sound and Facsimile by Artztand Foster. Reprint from RCA Review, No. 298, pages 89, 90, and 96.

